Home > Articles > Web Development

This chapter is from the book

This chapter is from the book

9.6 Polymorphic has_many Relationships

Rails gives you the ability to make one class belong_to more than one type of another class, as eloquently stated by blogger Mike Bayer:

  • The “polymorphic association,” on the other hand, while it bears some resemblance to the regular polymorphic union of a class hierarchy, is not really the same since you’re only dealing with a particular association to a single target class from any number of source classes, source classes which don’t have anything else to do with each other; i.e. they aren’t in any particular inheritance relationship and probably are all persisted in completely different tables. In this way, the polymorphic association has a lot less to do with object inheritance and a lot more to do with aspect-oriented programming (AOP); a particular concept needs to be applied to a divergent set of entities which otherwise are not directly related. Such a concept is referred to as a cross cutting concern, such as, all the entities in your domain need to support a history log of all changes to a common logging table. In the AR example, an Order and a User object are illustrated to both require links to an Address object.9

In other words, this is not polymorphism in the typical object-oriented sense of the word; rather, it is something unique to Rails.

9.6.1 In the Case of Models with Comments

In our recurring time and expenses example, let’s assume that we want both BillableWeek and Timesheet to have many comments (a shared Comment class). A naive way to solve this problem might be to have the Comment class belong to both the BillableWeek and Timesheet classes and have billable_week_id and timesheet_id as columns in its database table.

1 class Comment < ActiveRecord::Base
2   belongs_to :timesheet
3   belongs_to :expense_report
4 end

I call that approach naive because it would be difficult to work with and hard to extend. Among other things, you would need to add code to the application to ensure that a Comment never belonged to both a BillableWeek and a Timesheet at the same time. The code to figure out what a given comment is attached to would be cumbersome to write. Even worse, every time you want to be able to add comments to another type of class, you’d have to add another nullable foreign key column to the comments table.

Rails solves this problem in an elegant fashion by allowing us to define what it terms polymorphic associations, which we covered when we described the polymorphic: true option of the belongs_to association in Chapter 7, “Active Record Associations.” The Interface

Using a polymorphic association, we need to define only a single belongs_to and add a pair of related columns to the underlying database table. From that moment on, any class in our system can have comments attached to it (which would make it commentable) without needing to alter the database schema or the Comment model itself.

1 class Comment < ActiveRecord::Base
2   belongs_to :commentable, polymorphic: true
3 end

There isn’t a Commentable class (or module) in our application. We named the association :commentable because it accurately describes the interface of objects that will be associated in this way. The name :commentable will turn up again on the other side of the association:

1 class Timesheet < ActiveRecord::Base
2   has_many :comments, as: :commentable
3 end
5 class BillableWeek < ActiveRecord::Base
6   has_many :comments, as: :commentable
7 end

Here we have the friendly has_many association using the :as option. The :as marks this association as polymorphic and specifies which interface we are using on the other side of the association. While we’re on the subject, the other end of a polymorphic belongs_to can be either a has_many or a has_one and work identically. The Database Columns

Here’s a migration that will create the comments table:

1 class CreateComments < ActiveRecord::Migration
2   def change
3     create_table :comments do |t|
4       t.text :body
5       t.integer :commentable
6       t.string  :commentable_type
7     end
8   end
9 end

As you can see, there is a column called commentable_type, which stores the class name of associated object. The Migrations API actually gives you a one-line shortcut with the references method, which takes a polymorphic option:

1 create_table :comments do |t|
2   t.text :body
3   t.references :commentable, polymorphic: true
4 end

We can see how it comes together using the Rails console (some lines omitted for brevity):

>> c = Comment.create(body: 'I could be commenting anything.')
>> t = TimeSheet.create
>> b = BillableWeek.create
>> c.update_attribute(:commentable, t)
=> true
>> "#{c.commentable_type}: #{c.commentable_id}"
=> "Timesheet: 1"
>> c.update_attribute(:commentable, b)
=> true
>> "#{c.commentable_type}: #{c.commentable_id}"
=> "BillableWeek: 1"

As you can tell, both the Timesheet and the BillableWeek that we played with in the console had the same id (1). Thanks to the commentable_type attribute, stored as a string, Rails can figure out which is the correct related object. Has_many :through and Polymorphics

There are some logical limitations that come into play with polymorphic associations. For instance, since it is impossible for Rails to know the tables necessary to join through a polymorphic association, the following hypothetical code, which tries to find everything that the user has commented on, will not work:

 1 class Comment < ActiveRecord::Base
 2   belongs_to :user # author of the comment
 3   belongs_to :commentable, polymorphic: true
 4 end
 6 class User < ActiveRecord::Base
 7   has_many :comments
 8   has_many :commentables, through: :comments
 9 end
11 >> User.first.commentables
12 ActiveRecord::HasManyThroughAssociationPolymorphicSourceError: Cannot
13 have a has_many :through association 'User#commentables' on the polymorphic object

If you really need it, has_many :through is possible with polymorphic associations but only by specifying exactly what type of polymorphic associations you want. To do so, you must use the :source_type option. In most cases, you will also need to use the :source option, since the association name will not match the interface name used for the polymorphic association:

1 class User < ActiveRecord::Base
2   has_many :comments
3   has_many :commented_timesheets, through: :comments,
4            source: :commentable, source_type: 'Timesheet'
5   has_many :commented_billable_weeks, through: :comments,
6            source: :commentable, source_type: 'BillableWeek'
7 end

It’s verbose, and the whole scheme loses its elegance if you go this route, but it works:

>> User.first.commented_timesheets.to_a
=> [#<Timesheet ...>]

InformIT Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from InformIT and its family of brands. I can unsubscribe at any time.


Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about products and services that can be purchased through this site.

This privacy notice provides an overview of our commitment to privacy and describes how we collect, protect, use and share personal information collected through this site. Please note that other Pearson websites and online products and services have their own separate privacy policies.

Collection and Use of Information

To conduct business and deliver products and services, Pearson collects and uses personal information in several ways in connection with this site, including:

Questions and Inquiries

For inquiries and questions, we collect the inquiry or question, together with name, contact details (email address, phone number and mailing address) and any other additional information voluntarily submitted to us through a Contact Us form or an email. We use this information to address the inquiry and respond to the question.

Online Store

For orders and purchases placed through our online store on this site, we collect order details, name, institution name and address (if applicable), email address, phone number, shipping and billing addresses, credit/debit card information, shipping options and any instructions. We use this information to complete transactions, fulfill orders, communicate with individuals placing orders or visiting the online store, and for related purposes.


Pearson may offer opportunities to provide feedback or participate in surveys, including surveys evaluating Pearson products, services or sites. Participation is voluntary. Pearson collects information requested in the survey questions and uses the information to evaluate, support, maintain and improve products, services or sites, develop new products and services, conduct educational research and for other purposes specified in the survey.

Contests and Drawings

Occasionally, we may sponsor a contest or drawing. Participation is optional. Pearson collects name, contact information and other information specified on the entry form for the contest or drawing to conduct the contest or drawing. Pearson may collect additional personal information from the winners of a contest or drawing in order to award the prize and for tax reporting purposes, as required by law.


If you have elected to receive email newsletters or promotional mailings and special offers but want to unsubscribe, simply email information@informit.com.

Service Announcements

On rare occasions it is necessary to send out a strictly service related announcement. For instance, if our service is temporarily suspended for maintenance we might send users an email. Generally, users may not opt-out of these communications, though they can deactivate their account information. However, these communications are not promotional in nature.

Customer Service

We communicate with users on a regular basis to provide requested services and in regard to issues relating to their account we reply via email or phone in accordance with the users' wishes when a user submits their information through our Contact Us form.

Other Collection and Use of Information

Application and System Logs

Pearson automatically collects log data to help ensure the delivery, availability and security of this site. Log data may include technical information about how a user or visitor connected to this site, such as browser type, type of computer/device, operating system, internet service provider and IP address. We use this information for support purposes and to monitor the health of the site, identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents and appropriately scale computing resources.

Web Analytics

Pearson may use third party web trend analytical services, including Google Analytics, to collect visitor information, such as IP addresses, browser types, referring pages, pages visited and time spent on a particular site. While these analytical services collect and report information on an anonymous basis, they may use cookies to gather web trend information. The information gathered may enable Pearson (but not the third party web trend services) to link information with application and system log data. Pearson uses this information for system administration and to identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents, appropriately scale computing resources and otherwise support and deliver this site and its services.

Cookies and Related Technologies

This site uses cookies and similar technologies to personalize content, measure traffic patterns, control security, track use and access of information on this site, and provide interest-based messages and advertising. Users can manage and block the use of cookies through their browser. Disabling or blocking certain cookies may limit the functionality of this site.

Do Not Track

This site currently does not respond to Do Not Track signals.


Pearson uses appropriate physical, administrative and technical security measures to protect personal information from unauthorized access, use and disclosure.


This site is not directed to children under the age of 13.


Pearson may send or direct marketing communications to users, provided that

  • Pearson will not use personal information collected or processed as a K-12 school service provider for the purpose of directed or targeted advertising.
  • Such marketing is consistent with applicable law and Pearson's legal obligations.
  • Pearson will not knowingly direct or send marketing communications to an individual who has expressed a preference not to receive marketing.
  • Where required by applicable law, express or implied consent to marketing exists and has not been withdrawn.

Pearson may provide personal information to a third party service provider on a restricted basis to provide marketing solely on behalf of Pearson or an affiliate or customer for whom Pearson is a service provider. Marketing preferences may be changed at any time.

Correcting/Updating Personal Information

If a user's personally identifiable information changes (such as your postal address or email address), we provide a way to correct or update that user's personal data provided to us. This can be done on the Account page. If a user no longer desires our service and desires to delete his or her account, please contact us at customer-service@informit.com and we will process the deletion of a user's account.


Users can always make an informed choice as to whether they should proceed with certain services offered by InformIT. If you choose to remove yourself from our mailing list(s) simply visit the following page and uncheck any communication you no longer want to receive: www.informit.com/u.aspx.

Sale of Personal Information

Pearson does not rent or sell personal information in exchange for any payment of money.

While Pearson does not sell personal information, as defined in Nevada law, Nevada residents may email a request for no sale of their personal information to NevadaDesignatedRequest@pearson.com.

Supplemental Privacy Statement for California Residents

California residents should read our Supplemental privacy statement for California residents in conjunction with this Privacy Notice. The Supplemental privacy statement for California residents explains Pearson's commitment to comply with California law and applies to personal information of California residents collected in connection with this site and the Services.

Sharing and Disclosure

Pearson may disclose personal information, as follows:

  • As required by law.
  • With the consent of the individual (or their parent, if the individual is a minor)
  • In response to a subpoena, court order or legal process, to the extent permitted or required by law
  • To protect the security and safety of individuals, data, assets and systems, consistent with applicable law
  • In connection the sale, joint venture or other transfer of some or all of its company or assets, subject to the provisions of this Privacy Notice
  • To investigate or address actual or suspected fraud or other illegal activities
  • To exercise its legal rights, including enforcement of the Terms of Use for this site or another contract
  • To affiliated Pearson companies and other companies and organizations who perform work for Pearson and are obligated to protect the privacy of personal information consistent with this Privacy Notice
  • To a school, organization, company or government agency, where Pearson collects or processes the personal information in a school setting or on behalf of such organization, company or government agency.


This web site contains links to other sites. Please be aware that we are not responsible for the privacy practices of such other sites. We encourage our users to be aware when they leave our site and to read the privacy statements of each and every web site that collects Personal Information. This privacy statement applies solely to information collected by this web site.

Requests and Contact

Please contact us about this Privacy Notice or if you have any requests or questions relating to the privacy of your personal information.

Changes to this Privacy Notice

We may revise this Privacy Notice through an updated posting. We will identify the effective date of the revision in the posting. Often, updates are made to provide greater clarity or to comply with changes in regulatory requirements. If the updates involve material changes to the collection, protection, use or disclosure of Personal Information, Pearson will provide notice of the change through a conspicuous notice on this site or other appropriate way. Continued use of the site after the effective date of a posted revision evidences acceptance. Please contact us if you have questions or concerns about the Privacy Notice or any objection to any revisions.

Last Update: November 17, 2020